Photovoltaic devices, i.e., solar cells, are capable of converting solar radiation into usable electrical energy. The energy conversion occurs as the result of what is known as the photovoltaic effect. Solar radiation impinging on a solar cell and absorbed by an active region of semiconductor material generates electrons and holes. Electrons and holes are separated by a built-in electric field in the solar cell, such as a rectifying junction. The separation of electrons and holes by the rectifying junction results in the generation of an electric current known as the photocurrent and a voltage known as the photo-voltage. The electrons flow toward the region of semi-conductor material having N-type conductivity, and the holes flow toward a region of semiconductor material of opposite conductivity or a metal layer.
Basic types of hydrogenated amorphous silicon solar cells incorporating an intrinsic or undoped region are disclosed in U.S. Pat. No. 4,064,521, incorporated herein by reference. Hydrogenated amorphous silicon solar cells incorporating a halogen in the undoped region are disclosed in Application Ser. No. 727,659, filed Sept. 29, 1976, also incorporated herein by reference. The undoped hydrogenated amorphous silicon of the above types is slightly N-type when fabricated at an optimum substrate temperature of from about 250.degree. C. to about 350.degree. C. The slightly N-type nature of the undoped film under illumination implies a donor-like defect in the intrinsic hydrogenated amorphous silicon region. The space charge region of an amorphous silicon solar cell incorporating an undoped region could be increased if said region were not slightly N-type. Thus, it would be highly desirable to have an hydrogenated amorphous silicon solar cell with an increased width of the space charge layer under illumination and an undoped region which was not slightly N-type, but more nearly zero or substantially neutral under illumination.